2 questions are there more then 3 elbows in your vent run
and is any of it flexible pipe?
Humidity will infiltrate but should be infiltrating that much. One thing they make for dryer vents is a flap that opens one way to let air out, while dryer is running and closes when dryer is off. I don't think its humidity causing your issue. When you say the vent is straight does it run horizontal (even with the floor) or does elbow down and then run straight/horizontal?
Also about how long is the pipe run?
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